Deer hunting sport has been practiced for many centuries. Better materials, better weapon designs and ammunitions allow hunters to engage targets at longer ranges and with more precision than was possible in the past. Typical rifles such as Remington® model 243 allow engagements to ranges greater than 400 meters. However, it has been recognized that there are a number of factors that affect the position of the projectile hit point. Two dominant sources of hit point errors are the uncertainty in the estimation of the magnitude and direction of path integrated crosswind and the range to target. The effect of these errors on the hit point grows substantially as the standoff range between the hunter and the target increases and/or the crosswind increases. FIG. 1 shows the effect of uncompensated average crosswind and imprecise range measurement on hit probability for a 12 inch diameter target, as a function of range. The graph shows that for an average downrange crosswind of approximately 5 miles per hour (mph) and a 10% range uncertainty, the 243 caliber bullet misses the target aim point at 400 meters by over 13 cm. The effect is much worse at longer ranges, for instance, the bullet misses the target by 31 cm at a range of 600 meters, while missing the target by 63 cm at a range of 800 meters. Because a hunter cannot easily and accurately estimate the average wind and range to the target, there is a reduced probability of a first round target hit. During the day, an experienced user can estimate the crosswinds by viewing the mirage through the riflescope or the vegetation motion and the range to target by comparing the target size to the crosshair reticle but is unable to conduct these functions during the night or in twilight. Improved methods implemented include an anemometer at the hunter's location to estimate local winds and a laser range finder to estimate the range. If wind and range estimates were available, a ballistics calculator may then be used to calculate the wind and vertical hold offset coordinates. Even these advancements are inadequate. The anemometers just measure the local winds, while handheld range finders are difficult to keep on target, providing inaccurate results. Downrange winds can be significantly different than local winds; they can be non-uniform and change direction and/or amplitude along the flight path. These changes can be due to causes such as non-uniform terrain channeling and environmental pressure or temperature differential changes. For example, the effect of downrange winds on the hit point may be negligible if the crosswinds of same amplitude are blowing in one direction for one part of the path and in the opposite direction for another part of the path cancelling the overall projectile deviation. Because the local wind sensor cannot measure downrange winds, it provides an offset that would lead to a target miss.
Recent art, as disclosed in US Patent Application Publication No. 2013/0206836 A1, teaches the use of various forms of internal or external wind sensors at the user's position; all of which measure local winds. The assumption made in the previous art is that the downrange crosswinds are the same as measured by the local wind sensor. Experienced users know that this assumption is inaccurate because the projectile in flight integrates the winds as it flies along its trajectory to the target.
US Patent Application Publication No. 2013/0206836 A1 teaches the option of using LIDAR or laser Doppler Anemometry (or velocimetry) for wind measurement. The LIDAR method cannot easily measure projectile path crosswinds unless measurements are made in three known off-axial directions and the path-average crosswind calculated from the vector addition. This means that the measurement is not made close to the path the projectile travels. In addition the system requires impractical laser powers to achieve high accuracy at even modest ranges because the back-scattered signal modulated from aerosols in the atmosphere is approximately 6 orders of magnitude smaller than a modulated signal scattered from a solid target surface. Clear days, with high visibility to 23 km, can further reduce the range of engagement. This imposes stringent demands on required laser power, laser current drivers, power supply and signal processing, making the system size too big for practical mounting on the weapon. The Laser Doppler Anemometry approach to measuring winds involves detecting the scatter from particulates passing through a small volume generated at the intersection of two interfering laser beams. It is therefore a point measurement, and does not provide path-integrated wind from the shooter to the target.
Downrange path-integrated crosswind measurements from the shooter to the target are necessary to accurately predict the hit point of a projectile. Because the opportunity to engage and hit the target is time sensitive, all measurements must be done in near real time to calculate and display the offset aim point (OAP) in the user's sight; otherwise the opportunity may be permanently lost.
Other prior art, as disclosed in U.S. Pat. No. 8,196,828, proposes to measure downrange integrated crosswind using a laser collimated beam, single aperture and a single imager. In this approach, a high speed camera is used to image the laser spot on the target with a frame rate high enough to freeze the motion of the time varying scintillation pattern. The outbound laser beam is modulated by the atmospheric turbulence producing a time varying pattern of light and dark spots on the target that move and change with the wind. By measuring the time-lag covariance of geometrically-related pixel pairs in a series of recorded camera frames, the path-averaged crosswind can be calculated. This approach suffers from several drawbacks, including: 1) the effect of the return path turbulence on the signal scattered from the target acts as a noise source reducing the overall signal to noise ratio; 2) because the ability to resolve the light and dark spots on the target is limited by diffraction of the collecting lens aperture, large lens apertures (in excess of 100 mm) are required thus increasing the size of the system; 3) higher laser signal power is required because the returned signal spreads over many pixels due to aperture diffraction requiring higher optical power per pixel to measure the crosswind, thus significantly affecting battery life; 4) the approach is sensitive to the refractive index structure constant, Cn2 which reduces the size of the dark and light spots at values exceeding 10−13, requiring even higher optical resolution (i.e., an aperture larger than 100 mm and more optical power).
Other prior art, U.S. Pat. No. 8,279,287 and U.S. Patent Application Publication No. 2010/0128136, propose to measure downrange path integrated crosswind using a passive method. The technique uses at least two apertures with each aperture passively imaging the target without active light illumination. The atmospheric turbulence modulates the image of the target which appears wavy due to low-frequency wind motion. Using block matching processing approach, the transit time difference in the waviness of a single or multiple features from the two images of the target is measured to deduce the path-integrated crosswind. The approach requires multiple high contrast features on the target or sharp target edge that must first be identified using an imaging sensor and then processed to measure the time difference. Uniform targets without features or that blend into the background (camouflaged) cannot be resolved easily. To resolve the target features (approximately 1 cm) at 1 km, diffraction limited lens diameter of approximately 150 mm at visible wavelengths is required. The size of two such lenses makes the device impractical for mounting on a weapon.
Another approach is described in the article by Wang et al., “Wind measurement by the temporal cross-correlation of the optical scintillations,” Applied Optics V20, No. 23, December 1981. This article describes a breadboard system for measuring the path averaged crosswind configured such that a laser source at one end transmits light through the atmospheric turbulence and is detected by a pair of side by side optical receivers located at the other end. This one-way transmission system method can measure path-integrated average crosswinds using several processing techniques. All of these processing techniques are based on observing the wind-driven motion of the scintillation pattern that transits across the line of sight. For the hunting application, the one-way transmission system is clearly impractical because the laser and optical receivers must both be on the same side (user's end) of the path.
When adapting this one-way transmission system to a two-way reflective system, one of the key problems encountered is the laser speckle noise generated from the illuminated target. Laser speckle is an interference effect that creates non-uniform distribution of the light intensity (light and dark spots) when laser light reflects back from a target surface that has a surface roughness smaller than the coherence length of the laser. The speckle problem does not exist in the one-way system because light does not scatter from a target. In the two-way case, the laser light is scattered from the target and collected by the receivers located near the light source. Speckles generated at the target and reflected back appear similar to the scintillation pattern signal, which is created by atmospheric turbulence and used for measuring winds. As a result the covariance function is disturbed by the interference from speckle effects causing large errors in the wind measurement. To address this problem, a laser source with a short coherence length, compared to the target roughness, is required.
Because the aforementioned article by Wang et al. described a field experiment, the system disclosed therein did not have any size, weight and power constraints to meet. Any practical weapon mounted device, demands a compact size that can be operated for extended periods on one battery charge. As the diameter of the receiver lens decreases to allow a more compact system package, the received signal level goes down and aperture diffraction spreads the focused image over a larger area (a higher number of pixels if a camera receiver is used) which results in reduced SNR per pixel even if the total energy over all pixels is summed. This limits the size of the receiver lens that can be used. In the same way, if a laser divergence of 100 micro radians is required to ensure that a laser spot appears on the target at maximum range, the diffraction limits the minimum achievable lens diameter at that wavelength. Large transmitter and receiver apertures impose size constraints in designing a weapon mounted or portable compact system package.
In designing a compact system to measure a path-weighted average crosswind and a range-to-target, it would be advantageous to provide the user with an offset aim point (OAP) indicator in the sight that considers the second order effects from other variables such as: temperature, pressure, humidity, rifle-cant and tilt, ammunition type, etc. Sensors to measure these parameters should be small enough to not impact the size of the package significantly. Furthermore the package must be rugged enough to withstand the shock from repeated weapon firings. These constraints impose yet more challenges in the innovation of a small and portable system useful for operation on or off a weapon.